1. Field of the Invention
The present invention relates to a sequence of a wireless communication system, and more particularly to a method for establishing a cyclic shift in consideration of characteristics of a CAZAC sequence in order to solve the problem of a frequency offset.
2. Discussion of the Related Art
A Constant Amplitude Zero Auto-Correlation (CAZAC) sequence is a representative one of various sequences which have been intensively discussed in the 3GPP LTE.
Channels generally extract a variety of identifiers (IDs) or information using the CAZAC sequence, for example, synchronization channels (e.g., a primary-SCH, a secondary-SCH, and a BCH) for downlink synchronization, other synchronization channels (e.g., a RACH) for uplink synchronization, and pilot channels (e.g., a data pilot, and a channel quality pilot). Also, the above-mentioned CAZAC sequence has been used to perform the scrambling.
Two kinds of methods have been used for the CAZAC sequence, i.e., a first method for changing a root index to another, and employing the changed root index, and a second method for performing a cyclic shift (CS) on a single root sequence, and employing the CS-result.
If a current root index is changed to a new root index, a low cross-correlation occurs between the current root index and the new root index, however, there is no limitation in designing sequence usages.
In the case of the cyclic shift, zero cross-correlation exists between the current root index and the new root index, so that the two root indexes are used when each of the root indexes require a high rejection ratio. Specifically, when time-frequency resources are shared in the same cell and data/control signals are transmitted, the above-mentioned two root indexes are adapted to discriminate among different signals or UEs.
A representative example of CAZAC sequences is a Zadoff-Chu (ZC) sequence, and the Zadoff-Chu sequence can be defined by the following equation 1:
                                                        x              u                        ⁡                          (              n              )                                =                                    exp              ⁡                              (                                                      ju                    ⁢                                                                                  ⁢                                          πn                      ⁡                                              (                                                  n                          +                          1                                                )                                                                                                  N                    ZC                                                  )                                      ⁢                                                  ⁢            for            ⁢                                                  ⁢            odd            ⁢                                                  ⁢                          N              ZC                                      ⁢                                  ⁢                                            x              u                        ⁡                          (              n              )                                =                                    exp              ⁡                              (                                                      ju                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                                          n                      2                                                                            N                    ZC                                                  )                                      ⁢                                                  ⁢            for            ⁢                                                  ⁢            even            ⁢                                                  ⁢                          N              ZC                                                          [                  Equation          ⁢                                          ⁢          1                ]            
where “n” is indicative of a sampling index, “Nzc” is indicative of the length of the ZC sequence, and “u” is indicative of the root index of the ZC sequence.
However, if the offset occurs in a frequency domain in the same manner as in the case where the CAZAC sequence is transmitted using the OFDM scheme, a performance or false alarm or throughput may be excessively deteriorated.
Specifically, if the cyclic shift (CS) is applied to the CAZAC sequence, the frequency offset or the timing offset excessively occurs, so that it is difficult to discriminate between sequences.